1. Field of the Invention
The present invention relates to electric double layer capacitors comprising carbonaceous electrodes soaked in an organic electrolytic solution.
2. Description of the Related Art
Capacitors can repeat charge and discharge with a great amount of electric current and are hopeful for use as power storage devices accompanied with frequent charge and discharge. Capacitors, therefore, have been desired to be improved with respect to energy density, rapid charge-discharge characteristics, durability, etc.
The fact that carbonaceous electrodes are soaked in an organic electrolytic solution to form an electric double layer capacitor is known. Michio Okamura “Electric Double Layer Capacitors and Power Storage Systems” 2nd Edition, The Nikkan Kogyo Shimbun, Ltd., 2001, pages 34 to 37 discloses an electric double layer capacitor comprising a bath partitioned into two sections with a separator, an organic electrolytic solution filled in the bath and two carbonaceous electrodes, one electrode being soaked in one section of the bath and the other electrode being soaked in the other section of the bath. The organic electrolytic solution is a solution containing a solute dissolved in an organic solvent. The document discloses tetraethylammonium tetrafluoroborate (Et4NBF4) and the like as the solute and propylene carbonate as the solvent. As the carbonaceous electrodes, activated carbon is employed. The activated carbon refers to amorphous carbon which has a very large specific surface area because it has innumerable minute pores. In the present specification, amorphous carbon having a specific surface area not less than about 1000 m2/g is called activated carbon.
Japanese Patent Laid-open Publication No. H11(1999)-317333 discloses a nonporous carbonaceous material as carbonaceous electrodes for use in electric double layer capacitors. The carbonaceous material comprises fine crystalline carbon similar to graphite and has a specific surface area not larger than 300 m2/g, which is smaller than that of activated carbon. Nonporous carbonaceous electrodes are considered to produce capacitance with the mechanism completely different from that of carbonaceous electrodes composed of activated carbon. It is believed that application of voltage makes electrolyte ions intercalate with solvent between layers of fine crystalline carbon similar to graphite, resulting in formation of an electric double layer.
Japanese Patent Laid-open Publication No. 2002-25867 discloses production of carbonaceous electrodes using needle coke or infusibilized pitch as a raw material. The needle coke refers to calcined coke with high graphitizability which has well-developed needle-form crystals. Needle coke possesses high electrical conductivity and extremely low proportion of thermal expansion. It also has high anisotropy based on its graphite crystal structure. Needle coke is generally produced by a delayed coking system using specially-treated coal tar pitch or petroleum heavy oil as a raw material.
Japanese Patent Laid-open Publication No. 2000-77273 discloses an electric double layer capacitor including nonporous carbonaceous electrodes soaked in an organic electrolytic solution. The organic electrolytic solution must have ion conductivity, and therefore the solute is a salt composed of a cation and an anion combined together. As the cation, lower aliphatic quaternary ammonium, lower aliphatic quaternary phosphonium, imidazolinium and the like are described. As the anion, tetrafluoroboric acid, hexafluorophosphoric acid and the like are described. The solvent of the organic electrolytic solution is a polar aprotic organic solvent. Specifically, ethylene carbonate, propylene carbonate, γ-butyrolactone, sulfolane and the like are disclosed.
The nonporous carbonaceous electrodes show electrostatic capacitance several times as much as those shown by porous electrodes made from activated carbon, and also has characteristics of expanding during electric field activation. When carbonaceous electrodes expand, the volume of the capacitor itself also increases. Thus, the increment of electrostatic capacitance per unit volume is lessoned and it is impossible to increase the energy density of the capacitor sufficiently.
It is possible to reduce the expansion of the capacitor itself by mechanically pressing the carbonaceous electrodes beforehand. However, when the carbonaceous electrodes have high expansion proportion (empirically, an expansion proportion more than about 150%), great load is applied to a container of the capacitor. This causes difficulty in maintaining sealability of the container, and also reduction in its durability. Thus, the expansion proportion of nonporous carbonaceous electrodes during electric field activation has been required to be reduced as much as possible. Nonporous carbonaceous electrodes exhibit a great expansion particularly on the cathode side of a capacitor. The reduction of such expansion is effective for increase in energy density.
It is believed that the expansion of nonporous carbonaceous electrodes is caused by expansion of gaps of a layer structure of the electrodes, which gap expansion occurs when electrolyte ions intercalate between layers of fine crystalline carbon. It, therefore, is preferable that the diameter of electrolyte ions be small in order to reduce the expansion of nonporous carbonaceous electrodes. For this reason, electrolytes comprising ions having low molecular weight and simple structure have heretofore been used as solutes for organic electrolytic solutions of electric double layer capacitors.